add an optional E? assertion for documentation purposes

Author: AlexWaygood

conformance/results/mypy/directives_type_checking.toml

@@ -1,5 +1,6 @@
 conformant = "Pass"
 output = """
+directives_type_checking.py:22: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
 """
 conformance_automated = "Pass"
 errors_diff = """

conformance/results/pyrefly/directives_type_checking.toml

@@ -3,4 +3,5 @@ conformance_automated = "Pass"
 errors_diff = """
 """
 output = """
+ERROR directives_type_checking.py:22:14-16: `Literal['']` is not assignable to `int` [bad-assignment]
 """

conformance/results/pyright/directives_type_checking.toml

@@ -1,5 +1,7 @@
 conformant = "Pass"
 output = """
+directives_type_checking.py:22:14 - error: Type "Literal['']" is not assignable to declared type "int"
+  "Literal['']" is not assignable to "int" (reportAssignmentType)
 """
 conformance_automated = "Pass"
 errors_diff = """

conformance/results/ty/directives_type_checking.toml

@@ -2,4 +2,5 @@ conformance_automated = "Pass"
 errors_diff = """
 """
 output = """
+directives_type_checking.py:22:14: error[invalid-assignment] Object of type `Literal[""]` is not assignable to `int`
 """

conformance/results/zuban/directives_type_checking.toml

@@ -2,4 +2,5 @@ conformance_automated = "Pass"
 errors_diff = """
 """
 output = """
+directives_type_checking.py:22: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
 """

conformance/tests/directives_type_checking.py

@@ -16,3 +16,12 @@ def foo(x: list[int], y: list[str]) -> None:
         z = y
 
     assert_type(z, list[int])
+
+
+if TYPE_CHECKING:
+    x: int = ""  # E: In a `if TYPE_CHECKING` block, type checkers should report all errors as normal
+else:
+    x: int = ""  # E? Many type checkers suppress all errors in `else` blocks of `if TYPE_CHECKING`, though this is not currently specified
+
+if not TYPE_CHECKING:
+    x: int = ""  # E? Many type checkers suppress all errors in `if not TYPE_CHECKING` blocks, though this is not currently specified